As is known in the art, fiber optic endoscopy is typically conducted by transmitting an image through an array of fibers often referred to as a fiber bundle. While successful for a variety of medical and non-medical applications, utilization of an array of fibers to form the image imposes constraints on the cost, diameter, and flexibility of the imaging device. In an attempt to overcome these drawbacks, multiple approaches employing a single optical fiber have been proposed for miniature, flexible endoscopes. For example, one technique for confocal imaging with a single fiber has been implemented by utilizing the core of a single-mode fiber as both the source and the detection apertures. Also, miniature confocal microscope probes and endoscopes have been constructed by adding a mechanical micro-scanner at the tip of a single-mode fiber. Another single-fiber method for miniature endoscopy (referred to as spectral encoding) uses a broadband light source and a diffraction grating to spectrally encode reflectance across a transverse line within the sample.
In ophthalmic treatments it is common to use an optical beam to treat a patient's eye, for example, using visible laser light to treat diabetic retinopathy and age-related macular degeneration. Traditionally, adjustable optical beam diameters have been produced using a fixed light source with either a zoom lens or turret assembly to vary the magnification level. Alternatively, the optical beam may be defocused by changing the distance between the target and the last lens in the chain of optical elements to vary the beam spot size. While these techniques vary the beam spot size satisfactorily, they involve moving elements with large moments of inertia that lead to increases in fabrication costs and have speed limitations on beam spot adjustment.
To address certain of these shortcomings, other systems have been developed that have the ability to adjust beams, spot sizes and spot shapes, for example at the treatment plane to overcome some of the above-identified limitations. For example, U.S. Pat. No. 7,599,591 issued to D. E. Andersen et al. describes an optical delivery system and method for providing adjustable beam diameter, spot size and/or spot shape by modifying optical characteristics of beams, varying objects such as fibers or other optical elements, etc., to achieve final beam diameter of a desired size and shape.
As disclosed in other prior described systems, the core of the single-mode fiber acts as both the source and the detection apertures for these techniques. As is also known, one important design parameter for single-fiber endoscopy is the modal profile of the optical fiber. Single-mode optical fibers enable high resolution imaging with small and flexible imaging probes, but suffer from relatively poor light throughput. Furthermore, the small core of the single-mode fiber acts similarly to a pinhole in free-space confocal microscopy, preventing the detection of out-of-focus light. For endoscopic applications, this optical sectioning may not be desirable since a large depth of field, large working distance, and wide field of view are typically preferred. For endoscopic microscopy applications, optical sectioning may be sacrificed for increased light throughput. When illuminated by coherent sources, imaging via single-mode fibers also introduces so-called speckle noise, which significantly reduces the effective resolution and quality of the images.
Replacing the single-mode fiber with a relatively large diameter multi-mode optical fiber enables higher optical throughput and decreases speckle noise. Unfortunately, utilization of a large diameter multi-mode fiber severely deteriorates the system's point-spread function and prevents the use of interferometry for high sensitivity and three-dimensional detection. Recently, significant progress has been made developing high power fiber lasers utilizing double-clad (also called “dual-clad”) optical fibers. These fibers are unique in their ability to support single mode propagation through the core with multi-mode propagation through the inner cladding.
Therefore, a need exists for an improved technique of spot size adjustment utilizing multi-clad fibers.